Thực trạng véc tơ sốt xuất huyết dengue, mối tương quan giữa khí hậu với chỉ số véc tơ và số mắc sốt xuất huyết dengue tại 4 tỉnh miền bắc việt nam (2016 2017) tt tiếng anh

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INTRODUCTION
Dengue hemorrhagic fever (DHF) is an extremely
dangerous acute infection caused by Aedes mosquitoes, which
can be massively lethal if a major pandemic occurs. An estimate
of 500,000 people with severe dengue infection require
hospitalization each year, and about 2.5% of the cases die of the
disease. There hasn’t been specific treatment for dengue fever,
and vaccines are in the clinical trial stage so vector control is
very important for disease prevention.
In Vietnam, although there have been many efforts in active
prevention and control, dengue epidemics tend to increase and
expand its scope. The annual average number of cases is still very
high, about 70,000 - 100,000 with hundreds of deaths.
In the world, there have been many studies on the dengue
transmission role of dengue virus, as well as determination of
the affinity of dengue virus against Aedes aegypti and Aedes
albopictus. In Vietnam, few studies on this matter have been
raised; moreover Ae.aegypti and Ae.albopictus have different
biological, ecological and behavioral characteristics. What is the
correlation between climatic factors and the transmission of
dengue fever? Because of the above reasons, we conducted the
study: “Current situation of dengue vectors, the correlation
between climate, vector indices and dengue cases in 4 northern
provinces of Vietnam (2016 - 2017)” with the following 2
objectives:
1. Describe the distribution, resting habits, dengue transmission
role and insecticide susceptibility of Ae.aegypti and
Ae.albopictus in Hanoi, Hai Phong, Thanh Hoa and Ha Tinh,
2016 - 2017.

2. Analyze the correlation between climatic factors, vector
indices, and dengue cases in Hanoi, 2016 - 2017.
NOVELTY, SCIENTIFIC AND PRACTICAL
SIGNIFICANCE OF THE THESIS
1. The study has identified Ae. aegypti as a dengue
transmission agentive, specifically discovered Ae.albopictus
at the dengue outbreak in Vietnam.


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2. The study has identified the correlation between climatic
factors, vector indices, and dengue prevalence in Hanoi,
especially the close relation between average monthly
temperature, entomology indices and the number of cases.
This is useful information for forecasting the development
direction of dengue.

THESIS STRUCTURE
The thesis consists of 124 pages divided into the following
sections: Introduction (02 pages), literature review (32 pages),
study subjects and methods (20 pages), study results (36 pages),
discussions (32 pages), conclusions (02 pages), and
recommendations (1 page). There are 41 tables, 11 figures, and
117 references.
CHAPTER 1
LITERATURE REVIEW
1.1. Situation of dengue fever
1.1.1. Situation of dengue fever in the world
The first outbreak of dengue fever was recorded with a
clear causative agent in Australia in 1897, followed by Greece

in 1928 and Taiwan in 1931. Today, dengue outbreaks occur in
more than 100 countries in different territories from Africa,
America, the Middle East, Southeast Asia to the Western
Pacific, in which America, Southeast Asia and the Western
Pacific are the most infected.
1.1.2. Situation of dengue fever in Vietnam
First dengue outbreaks were recorded in Northern
Vietnam in 1958 and in the Mekong Delta in Southern Vietnam
in early 1960s. The situation of dengue fever in the last 3 years
(2015 - 2017) has increased sharply both in the number and
expansion. Especially, dengue no longer occurs in cities and
plains but has spread to highland and mountainous areas. In
2017, dengue outbreaks bloomed in many provinces and cities
nationwide, with a focus on Hanoi and Ho Chi Minh City.


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1.1.3. Situation of dengue fever at study sites
1.1.3.1. Situation of dengue fever in Hanoi
In the past 20 years, Hanoi has witnessed many big
dengue outbreaks such as in 1998 with the main cause is
dengue virus 3. In 2009, the number of cases across the North
was 18,485 cases, in which there were 16,090 cases and 4
deaths recorded in Hanoi, accounting for 87%. In 2015, Hanoi
had 15,412 cases. In 2017, dengue occurred throughout the city
with 37,651 cases and 7 deaths.
1.1.3.2. Situation of dengue fever in Hai Phong
Through surveillance, Hai Phong had 113 cases in 2015,
8 cases in 2016, and a sharp increase of 431 cases in 2017.
1.1.3.3. Situation of dengue fever in Thanh Hoa

The situation of dengue fever in Thanh Hoa increased
sharply with 171 cases recorded in 2016 and 3,374 cases in
2017.
1.1.3.4. Situation of dengue fever in Ha Tinh
The total number of dengue fever cases in 2016 was 39
with 0 deaths, which decreased by 33.8% compared to the
same period in 2015. In 2017, there were 194 cases and 0
deaths.
1.2. Causative agent of dengue fever
Dengue fever is an acute infectious disease caused by
Dengue virus. The disease can progress to cause death. Dengue
virus includes 4 types: Dengue 1 (D1), Dengue 2 (D2), Dengue
3 (D3) and Dengue 4 (D4).
1.3. Life cycle and morphology of Aedes mosquitoes
1.3.1. Life cycle of Aedes
The life cycle of Aedes includes 4 stages: Eggs - Larvae Pupae - Adult mosquitoes, in which the first 3 stages live in
water, only adult mosquitoes live on land.
1.3.2. Morphological characteristics of Aedes
1.3.2.1. Aedes eggs
The eggs of Aedes mosquitoes are under 1mm, oval in
shape, white at birth and then turning dark black.
1.3.2.2. Aedes larvae
The eighth abdominal segment of Aedes larvae is devoid
of any chitinous plates. The comb scales of the eighth
abdominal segment form a line, with middle and lateral spines
for Ae. aegypti and without lateral spines for Ae. albopictus.


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1.3.2.3. Aedes pupae

Aedes pupae are shaped like a large question mark
similar to other mosquito larvae.
1.3.2.4. Aedes adults
Aedes adults are easy to identify with distinct black and
white compartments in legs and abdomens, and silvery scales
on bodies concentrated in clusters or lines.
1.4. Distribution and habits of Aedes
1.4.1. Distribution of Aedes
1.4.1.1. Distribution of Ae. aegypti and Ae. albopictus in the
world
In the first half of the 20th century, Ae. aegypti was
mostly found in tropical and subtropical regions between
latitude 450 North and latitude 340 South across Asia, America
and Africa. Ae. aegypti is widely distributed in South and
Central America.
Ae. albopictus is currently classified as the most invasive
mosquito species and they are distributed on many continents:
Asia, America, Europe and Africa. This species is widely
distributed in the tropics and subtropics of continents mainly
located from 350 north latitude to 350 south latitude and to 450
north latitude limited by the 100C isotherm.
1.4.1.2. Distribution of Aedes aegypti and Aedes albopictus in
Vietnam
Ae. aegypti is found in most cities, towns, rural areas and
even in mountainous and highland areas.
Ae. albopictus is commonly found in the Northern
region. In recent years, it is also found in the South, Central
region and Central Highlands.
1.4.2. Habits of Aedes
1.4.2.1. Resting habits

Ae. aegypti has indoor resting habits. Meanwhile, Ae.
albopictus mainly lives outdoors, hiding under shrubs near or
away from houses.
1.4.2.2. Biting habits
Ae. aegypti is usually collected in daytime.


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1.4.2.3. Oviposition habits
Domestic studies have shown that Ae. aegypti larvae
were found in artificial water containers in and around houses.
Ae. albopictus often lays eggs in clean natural water.
1.5. Disease transmission role of Ae. aegypti and

Ae. albopictus

1.5.1. Disease transmission role of Aedes in the world
Studies have shown that the percentage of Ae. Aegypti caught
at study sites and location of dengue outbreaks that was positive for
Dengue virus ranged from 1.33% to 12.7%. For Ae. albopictus, this
rate was from 2.9% to 11.76%.
1.5.2. Disease transmission role of Aedes in Vietnam
The study on dengue outbreaks by Vu Sinh Nam (1995)
showed that all active dengue outbreaks contained Ae. Aegypti;
few outbreaks had both species, including a very small
percentage of Ae. albopictus.
Tran Van Tien (2003) showed that Ae. albopictus was
widely present in many localities and different populated areas,
especially in suburban areas covered with many green trees.
Meanwhile, Ae. aegypti is often found in urbanized areas and

inner cities with dense populations. Vu Trong Duoc (2012)
identified the role of Ae. aegypti and Ae. albopictus in some
outbreaks in Hanoi in 2011. Results showed that 10.4% of Ae.
aegypti at active outbreaks were infected with dengue virus,
however, dengue virus was not found in any Ae.albopictus at
outbreaks.
1.5.3. Correlation between vector density and dengue fever
situation
According to Vu Trong Duoc (2015), a study on the
relationship between Ae.aegypti and Ae.albopictus density with
dengue fever situation in Hanoi, 2011 - 2013, both Ae.aegypti
and Ae. Albopictus were present in an outbreak. In an active
outbreak, Ae.aegypti density was higher than that of Ae.
albopictus. In contrast, in areas without outbreaks, Ae.aegypti
density was much lower than that of Ae.albopictus.
1.6. Insecticide resistance of dengue vectors
1.6.1. Insecticide resistance of dengue vectors in the world
According to the announcement of the World Health
Organization, of the 200 arthropod species of medical
importance, up to 50% are dengue, malaria, filariasis


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transmission mosquitoes. Therefore, the determination of
insecticide resistance and resistance mechanisms for dengue
vectors is necessary to improve the effectiveness of the
national program for dengue prevention and control.
1.6.2. Insecticide resistance of dengue vectors in Vietnam
Ae.aegypti is resistant to DDT and most of the pyrethroid
insecticides in many places, but sensitive to malathion. Ae.

albopictus is still susceptible to insecticides.
1.7. Correlation between climatic factors and dengue fever
1.7.1. Studies on the correlation between climatic factors and
dengue fever in the world
Patz (1998) studied climatic factors and found that
temperature was associated with the spread of dengue fever.
Hales (1999) found that there was a link between the
prevalence of dengue fever and temperature, usually the
incubation period was shorter as temperatures rised. Poveda
(2000) showed that most of the peaks of the dengue epidemics
in Colombia corresponded to the El Nino + 1 phenomenon.
1.7.2. Studies on the correlation between climatic factors and
dengue fever in Vietnam
Tsuzuki (2009) with the study on transmission risks of
dengue fever during summer in Nha Trang city found that less
water containers resulted in a reduction in mosquito breeding
sites, leading to a lower risk of dengue transmission. Hoang
Thuy Nguyen (1994) with the study on the dengue situation
showed that mosquito density increased during the rainy
season.
CHAPTER 2

STUDY SUBJECTS AND METHODS
2.1. Study subjects
- Ae. aegypti and Ae. Albopictus larvae and mosquitoes
- Dengue virus.
- Climatic factors: Climatic data were taken from the
National Centre for Hydro-Meteorological Forecasting.
- Dengue cases: Data of dengue cases were taken from
Hanoi Preventive Medical Center.

2.2. Study duration
- From January 2016 to December 2017.


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2.3. Study sites
2.3.1. In the field
2.3.1.1. Cross-sectional survey
With the above selection criteria, 4 provinces/cities with
8 districts and 16 communes/wards were chosen.
2.3.1.2. Longitudinal survey
Longitudinal survey was conducted in Lang Thuong,
Lang Ha wards, Dong Da district and Tan Trieu, Tu Hiep
communes, Thanh Tri district, Hanoi.
2.3.1.3. Outbreak investigation
When a dengue outbreak was notified by the local
authorities, an entomological survey would be carried out at the
outbreak location of the four studied provinces/cities.
2.3.1.4. Determination of susceptibility of mosquitoes
Larvae collected from cross-sectional surveys and
outbreaks were preserved and brought to the laboratory for
rearing into adult mosquitoes. If the number of adult
mosquitoes was sufficient, susceptibility testing was
conducted.
2.3.2. In the laboratory
Testing was conducted in laboratories of Department of
Entomology, Department of Molecular Biology in the National
Institute of Malariology, Parasitology, and Entomology.
2.4. Study contents
- Survey of larval and mosquito indices of Ae.aegypti and

Ae.albopictus in 4 studied provinces.
- Assessment of the distribution characteristics of Aedes
mosquitoes by habitat and season.
- Description of resting habits of Aedes mosquitoes.
- Identification of the disease transmission role of Ae.
aegypti and Ae. albopictus at study sites.
- Evaluation of insecticide susceptibility of Aedes at study
sites.
- Analysis of the correlation between climatic factors and
vector indices of Ae. aegypti.
- Analysis of the correlation between climatic factors,
vector indices, and dengue cases.


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2.5. Study

methods
- Cross-sectional descriptive study: Cross-sectional
survey was onducted two times per year in 4 studied provinces/
cities.
- Laboratory research: Molecular biology tests were
conducted to determine the rate of dengue virus infection in
mosquitoes. Biological testing was carried out to assess the
insecticide susceptibility of mosquitoes.
- Longitudinal study: The survey to collect mosquitoes
and larvae was carried out monthly in the year in 4
communes/wards in Hanoi. Data of dengue cases and climate
in Hanoi were also collected by month to analyze and
determine the correlation between climatic factors, vector

indices, and dengue cases.
2.5.1. Sample size
- All mosquitoes and larvae collected from households.
- The number of households to be surveyed in accordance
with Decision No. 3711/QD-BYT.
- Sensitivity evaluation: According to the World Health
Organization.
- Household outbreak: 30 households in an outbreak.
- Investigated outbreaks: 117 outbreaks.
- Climatic factors of Hanoi, 2016 - 2017.
- Dengue cases in Hanoi, 2016 - 2017.
2.5.2. Sampling
2.5.2.1 For description of Aedes distribution
Households were selected randomly from the list.
2.5.2.2. For study of disease transmission role of Aedes
All adult mosquitoes collected indoors and outdoors by
Mosback.
2.5.2.3. For susceptibility testing
- Chemical impregnated papers according to the World
Health Organization standards included
5 types:
Alphacypermethrin 30mg/m2, deltamethrin 0,05%, permethrin
0,75%, lambdacyhalothrin 0,05% và malathion 5%.
2.5.2.4. For study of the correlation between climatic factors,
vector índices, and dengue cases.
- Climatic factors in Hanoi, 2016 - 2017.
- Mosquito density index, BI.
- Dengue cases in Hanoi, 2016 - 2017.

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2.6. Techniques used in the study
2.6.1. Techniques for entomological survey
2.6.1.1. Mosquito collection and preservation
2.6.1.2. Larva collection and preservation
2.6.2. Techniques for identification of disease transmission
role of Aedes
2.6.2.1. Dengue virus RNA purification by ARN adsorption
column using gene Jet RNA Purification (SOP
NIMPE.HD03.PP/18)
2.6.2.2. Identification of dengue virus in Ae. aegypti and
Ae. albopictus Multiplex reverse Transcriptase PCR (SOP
NIMPE.HD03.PP/19)
2.6.3. Techniques for evaluation of insecticide susceptibility
of mosquitoes by biological testing method.
2.7. Research indices
- Mosquito density index.
- House index (HI).
- Breteau index (BI).
- Container index (CI)
- Breteau index (HI).
2.8. Errors in research and how to fix it
2.9. Data input and analysis
2.9.1. Data input
Data input by Microsoft Excel software and all data
converted into SPSS format.
2.9.2. Data analysis
- Identify the percentage (%) of dengue virus in Ae.
aegypti and Ae. Albopictus larvae and mosquitoes.
- Use the correlation coefficient (r) to analyze the

correlation between climatic factors, vector indices and dengue
cases.
2.10. Data

processing

Analyze, process and present data of research results by
Exel and SPSS 16; compare the average value with the
statistical functions 2, t-test to determine the degree of
difference


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2.11. Ethical issues

The study strictly follows the regulations set forth
for biomedical research in the National Institute of
Malariology, Parasitology and Entomology.
CHAPTER 3

STUDY RESULTS
3.1. Dengue vectors at study sites
3.1.1. Distribution of Aedes at study sites
Study results showed that Ae. aegypti and Ae. albopictus
were mostly present at study sites in Hanoi, Hai Phong, Thanh
Hoa and Ha Tinh. During the cross-sectional survey, Ae. aegity
and Ae. albopictus larvae and adults were collected, but Ae.
aegypty larvae and adults were not surveyed in Hai Phong and
Thanh Hoa.

3.1.2. Resting habits of Aedes at study sites

3.1.2.1. Percentage of Ae. aegypti resting indoors and outdoors
Ae. aegypti was mainly active and resting indoors (94.5%).
Only 5.5% of the surveyed Ae. aegypti populations rested
outdoors.
3.1.2.2. Resting locations of Ae. aegypti were mainly
household living spaces
Table 3.18. Number and percentage of Ae. aegypti in
household living spaces
Living
Bedroom Kitchen Restroom
room
No. Location No. Perce No. Perce No. Perc No. Perc
(mos ntage (mos ntage (mos enta
(mos entag
ge
e
q.) (%) q.) (%) q.)
q.)
(%)
(%)
1 Hanoi
98 6.6 117
6 79.6 60 4.06 144 9.7
2 Hai Phong 30 9.7 236 76.4
7
2.3 36 11.7
3 Thanh Hoa 32 7.7 312 74.8 21 5.0 52 12.5
4 Ha Tinh

28 8.2 267 78.5
9
2.7 36 10.6
Total (%)
188 7.4 1991 78.3 97 3.8 268 10.5


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3.1.2.3. Resting heights of Ae. aegypti
Table 3.19. Percentage of Ae. aegypti resting at different
heights
Number of mosquitoes collected at different
heights
< 0.5 m
0.5 – 1
1.0 -2.0 m
>2m
No Location
Perc
.
No. Perce No. Perce No. Perce No. entag
(mos ntage (mos ntage (mosq. ntage (mos e
q.) (%) q.) (%)
)
(%) q.) (%)
1 Hanoi
145 9.4 208 13.5 1163 75.7 21 1.4
2 Hai Phong 67 20.4 54 16.5 198 60.4 9 2.7
3 Thanh Hoa 98 22.5 92 21.1 234 53.8 11 2.5
4 Ha Tinh

45 11.5 43 10.9 297 75.6 8 2.0
Total (%)
355 13.2 397 14.7 1892 70.3 49 1.8
3.1.2.4. Resting locations of Ae. aegypti
Most of Ae. aegypti (75.9%) were collected on clothes.
3.1.2.5. Percentage of Ae. albopictus resting indoors and outdoors
Table 3.21. Number of Ae. albopictus resting indoors and
outdoors at study sites
Trong nhà
Ngoài nhà
No.
Location
Total No. Percent No. Percent
(mo age (%) (mo age (%)
sq.)
sq.)
1
2
3
4

Hanoi
Hai Phong
Thanh Hoa
Ha Tinh
Total (%)

602
731
506

634
2473

20
23
30
22
95

3.3
3.1
5.9
3.5
3.8

582
708
476
612
2378

96.7
96.9
94.1
96.5
96.2

3.1.2.6. Resting locations of Ae. albopictus
Results showed that Ae. albopictus mostly rested around
larval sources outdoors (95.73%), followed by bedrooms

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(3.15%), living rooms (0.45%), restrooms (0.36%), and kitchen
(0.31%).
3.1.2.7. Resting heights of Ae. albopictus
Table 3.23. Percentage of Ae. albopictus resting at different
heights
< 0.5 m
0.5-1 m
1.0-2.0 m
>2m
Per
No.
Perce
No.
Perce
No.
Perce
No.
cen
No. Location
(mo ntage (mo ntage (mos ntage (mo tag
sq.) (%) sq.) (%) q.) (%) sq.) e
(%)
1 Hanoi
98 16.3 189 31.4 298 49.5 17 2.8
2 Hai
102 14.0 179 24.5 435 59.5 15 2.1
Phong

3 Thanh
87 17.2 145 28.7 267 52.8
7 1.4
Hoa
4 Ha Tinh 84 13.2 188 29.7 346 54.6 16 2.5
Total (%)
371 15.0 701 28.3 1346 54.4 55 2.2
3.1.2.8. Resting locations of Ae. albopictus
Ae. albopictus were mostly collected around larval
sources outdoors (86.00%), followed by clothes (7.5%), woods
(2.3%), bednets (2.3%), and walls (0.5%).
3.1.3. Dengue transmission role of Aedes
3.1.3.1. Number of Ae. aegypti and Ae. albopictus at outbreak
locations
Among 117 outbreaks investigated, 107 outbreaks
contained Ae. Aegypti, accounting for 91.5%; 71 outbreaks
contained Ae. albopictus accounting for 60.7% and 61 outbreaks
had both the species accounting for 52.1%. Hanoi had the
biggest number of outbreaks (71.8%), followed by Thanh Hoa
(12%), Hai Phong (8.5%), and Ha Tinh (7.7%).
3.1.3.2 Results of identifying Dengue virus on Ae. aegypti at
outbreak locations
Table 3.27. Results of identifying Dengue virus on Ae.
aegypti at outbreak locations, 2016 - 2017


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N Province/
o.

city
1
2
3
4
5 Hanoi

District

Perce Type
No.
ntage
of
of Positiv
(%)
dengue
outb e (+)
virus
reaks

Ba Dinh
Cau Giay
Dong Da
Ha Dong
Hai Ba Trung

2
2
13
18

12

Thanh Xuan
Thanh Tri
Nam Tu Liem
9 Hai Phong Cat Hai

16
8
7
8

10 Thanh Hoa Tinh Gia
Ha Tinh city
11
Ha Tinh
Thach Ha
12
Total

12
1

6
7
8

8

0

0
3
2
2
2
1
0
1
0
0
0
11

0.00
0.00
23.00
11.10
16.6
12.5
12.5
0.00
12.50
0.00
0.00
0,00

(-)
(-)
D1, D4
D1, D2

D1
D1, D3
D1
(-)
D1
(-)
(-)

(-)
107
10.28% D1, D2,
D3, D4
3.2.3.3. Percentage of Dengue virus on Ae. aegypti larvae at
outbreaks
None of 540 samples of Ae. aegypti larvae were found
positive for dengue virus.
3.1.3.4. Percentage of Dengue virus on Ae. albopictus at outbreaks
Ae. albopictus found at outbreaks was positive for
dengue virus; 2/71 outbreaks were infected with Dengue virus
type D1, accounting for 2.82%.
3.1.3.5. Percentage of Dengue virus on Ae. albopictus larvae at
outbreaks
A total of 88 samples of Ae. albopictus larvae were
detected at study sites. No samples were positive for dengue
virus.


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3.1.3.6. Percentage of Dengue virus on Ae. aegypti through
cross-sectional survey

At the sites of cross-sectional survey, no positive samples
were found.
3.1.3.7. Percentage of Dengue virus on Ae. albopictus larvae
and mosquitoes through cross-sectional survey
No positive samples were found.
3.1.4. Susceptibility of Aedes mosquitoes at study sites
3.1.4.1. Insecticide susceptibility of Ae. aegypti
Ae. aegypti was sensitive to alphacypermethrin at 3/13
points, possibly resistant at 1/13 points and resistant at 9/13
points; susceptable to deltamethrin at 4/13 points and resistant at
9/13 points; susceptable to lambdacyhalothrin at 3/13 points,
possibly resistant at 2/13 points, resistant at 8/13 points;
sensitive to permethrin at 1/13 points, possibly resistant at 3/13
points, resistant at 9/13 points; sensitive to malathion at 8/13
points, possibly resistant to 1/13 points, and resistant at 4/13
points.
3.1.4.2. Insecticide susceptibility of Ae. albopictus
Ae. albopictus was sensitive to alphacypermethrin at 12/23
points, possibly resistant at 5/23 points, and resistant at 6/23
points; sensitive to deltamethrin at 17/23 points, possibly
resistant at 1/23 points and resistant at 7/23 points; susceptable
to lambdacyhalothrin at 10/23 points, possibly resistant at 8/23
points and resistant at 5/23 points; susceptible to permethrin at
16/23 points, possibly resistant at 4/23 points and resistant at
3/23 sites; sensitive to malathion at 21/23 points, possibly
resistant at 1/23 points and resistant at 1/23 points.
3.2. Correlation between climatic factors, vectors and
dengue situation in Hanoi
3.2.1 Average temperature, humidity, rainfall in Hanoi
In 2016 and 2017, the average temperature of the

corresponding months was of no significant difference.
The average monthly rainfall in Hanoi was much
different between the rainy and dry season.
The humidity did not have much difference between
months in year.

3.2.2. Correlation between climatic factors and vector indices

Table 3.35. Correlation between climatic factors and Ae. aegypti
indices in Hanoi


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Factors
Average monthly
temperature
Average monthly humidity
Average monthly rainfall

Density
r
p
24 0.29 0.24
n

BI
r
0.55

p

0.006

24 -0.31 0.14 -0.22
0.31
24 0.30 0.16 0.37
0.07
P < 0.05
3.2.3. Correlation between climate, vectors and dengue cases
in Hanoi
Table 3.36. Correlation between climate, vectors and
dengue cases in Hanoi
Cases
Factors
n
r
p
Average monthly
24
0.24
0.27
temperature
Average monthly humidity
24
-0.03
0.89
Average monthly rainfall
24
0.19
0.38
Monthly mosquito density

24
0.39
0.06
Monthly BI
24
0.66
0.00
P < 0.05
3.2.4. Correlation between climate, vector indices, dengue
cases of the previous month and dengue cases of the
following month in Hanoi
Table 3.37. Correlation between climate, vector indices,
dengue cases of the previous month and dengue cases of the
following month in Hanoi
Cases
Factors
n
r
p
Average last month temperature
23
0.49
0.02
Average last month humidity
23
0.01
0.95
Average last month rainfall
23
0.39

0.06
Last month mosquito density
23
0.48
0.02
Last month BI
23
0.74
0.00
Last month cases
23
0.83
0.00
P < 0.05


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CHAPTER 4

DISCUSSIONS
4.1.

Situation of dengue vectors at study sites
4.1.1. Distribution of dengue vectors at study sites
Study results showed that Ae. aegypti and Ae. albopictus
were mostly present at study sites in Hanoi, Hai Phong, Thanh
Hoa and Ha Tinh. During cross-sectional investigation, Ae.
aegypti and Ae. albopictus larvae and mosquitoes were found
at the study sites, but no Ae. aegypty larvae and mosquitoes
found in Hai Phong and Thanh Hoa (tables 3.3 and 3.5).

However, their distribution was different at the study sites. The
study on the distribution of Ae.aegypti and Ae. albopictus at the
study sites in 2016 - 2017 showed that 94.5% of Ae.aegypti
were active and rested indoors; 96.2% of Ae.albopictus were
active and rested outdoors. This finding is similar to some
studies of determination of distribution of Aedes mosquitoes in
some Northern provinces in the past, typically Vu Sinh Nam
(1995). Common resting places of Ae.aegypti were dark,
windless such as hanging clothes, blankets, bednets, accounting
for over 90%. A small percentage of Ae.aegypti was seen on
the wall. Studies show that Ae.aegypti prefers to rest indoors,
their larvae live in artificial water containers, they live close to
people, so they are often present in densely populated urban
areas. In contrast, Ae.albopictus often rests outdoors with
larvae living in natural and artificial water containers.
4.1.2. Ecological characteristics of Aedes mosquitoes
Study results in Hanoi, Hai Phong, Thanh Hoa and Ha
Tinh showed that Ae.aegypti adults rested on clothes (77%),
mosquito nets (7.6%), clothesline (4.4%), wood furniture
(4.5%), and walls (1.6%). Only a small number of Ae.aegypti
(1.6%) rested on the wall, which is the reason why no residual
spraying on the wall is applied. When investigating
households, we found that, because of economic and social
conditions or habits of the people, clothes were hung directly
on the wall in poorly-ventilated rooms, which is a favorable
condition for Ae.aegypti.
Ae.aegypti lives in house close to people, but not every
location they are present. Their life span depends on natural
conditions such as temperature, humidity so they often rest in cool

17
places with high humidity, weak light density, and lots of
substrates for shelter. By daytime indoor light trap catching we
found that Ae.aegypti mainly rested in the bedroom (78.3%),
followed by toilets (10.5%), living room (7.4%), and kitchen
(3.8%), which is consistent with the habit of this species.
Ae.albopictus mainly lives and rests outdoors. Survey of the
households at the research sites showed that Ae.albopictus was
mostly present around outdoor larvae sources (95.73%), followed
by bedrooms (3.15%), and living rooms, toilets and kitchens
(0.45%, 0.36% and 0.31% respectively). Resting locations of
Ae.albopictus included objects around outdoor larva sources
(86.0%), clothes (7.5%), wood (2.3%), nets (2.3%), and walls
(0.5%).
4.1.3. Disease transmission role of Aedes
In order to prove that Aedes mosquitoes are responsible
for transmitting dengue fever, we have to determine their
presence in active outbreaks. The results in Table 3.25 indicate
that Ae.aegypti or Ae.albopictus or both was found in all the
outbreaks. Ae.aegypti found in the active outbreak accounted
for the highest percentage of 91.5%, followed by Ae.
albopictus (60.7%), and both (52.1%). From the data above, we
can see that all dengue outbreaks saw the presence of Aedes
mosquitoes, indicating that Aedes mosquitoes play a role in
transmitting dengue virus. Our study results are similar to those
of Vu Trong Duoc (2015) with the study of the dengue
transmission role of Aedes in Hanoi. His study showed that the
rate of Ae.aegypti at outbreaks was 94% of the total 2 species,
but this rate was only 28.5% by cross sectional survey. About

half of the active outbreaks reported the presence of only
Ae.aegypti but Ae. albopictus. Among the remaining outbreaks
containing both the species, Ae.aegypti overwhelmed
Ae.albopictus [13]. Our findings are somewhat different from
those of Vu Sinh Nam (1995), which said that most of the
dengue outbreaks had the presence of Ae.aegypti and only a
few outbreaks had both Ae.aegypti and Ae.albopictus [28]. In
this study we found 10.28% of Ae.aegypti positive for dengue
fever, 2.82% of Ae.albopictus positive for dengue fever at the
outbreaks. Although testing was not carried out on mosquito
individual, this result is quite consistent with some studies in
Vietnam and around the world. Typically, Vu Trong Duoc


18
(2012) has identified the role of Ae.aegypti and Ae.albopictus
in some dengue outbreaks in Hanoi in 2011. Results showed
that 10.4% of Ae.aegypti at active outbreak were infected with
dengue virus, however, dengue virus was not found in
Ae.albopictus[11].
4.1.4. Insecticide susceptibility of Aedes
Test results showed that Ae.aegypti in Hanoi was
resistant to alphacypermethrin, deltamethrin, permethrin,
lambdacyhalothrin at all study sites with mortality of 3 - 72%;
resistant to malathion at 4/5 study sites at a mortality rate of
59-73%; sensitive to this chemical in Phuong Liet ward, Dong
Da district (mortality 100%). Testing in Hai Phong showed that
Ae.aegypti in Cat Hai was resistant to alphacypermethrin,
deltamethrin, permethrin, lambdacyhalothrin (mortality from
67 - 88%); sensitive to alphacypermethrin, deltamethrin,

lambdacyhalothrin in Phu Long commune, Cat Hai district
(mortality rate from 98 to 100%), increased endurance with
permethrin with mortality rate of 90%. Ae.aegypti was
susceptable to malathion at 3/3 study sites (100% mortality). At
the study sites in Thanh Hoa, Ae. aegypti was also sensitive to
alphacypermethrin at 4/6 study sites (98% - 100% mortality);
resistant to this chemical in Hai Ha commune, Tinh Gia district
(mortality rate 72%); susceptable to lambdacyhalothrin at 1/7
sites (mortality 98%); increased tolerance to this chemical at
4/6 sites (mortality 90% - 94%); resistant to this chemical at
1/6 research point (mortality 88%); sensitive to permethrin 3/6
research points (98% - 100% mortality rate); increased
endurance with this chemical in Dong Hai ward, Thanh Hoa
city and Hai Ha commune, Tinh Gia district (92% and 94%
respectively); resistant to this chemical in Dong Tho ward,
Thanh Hoa city (mortality 88%); susceptable to malathion at
6/6 research sites (100% mortality). Ae. aegypti was sensitive
to alphacypermethrin at 1/3 sites (100% mortality); resistant to
this chemical at 1/3 sites (92% mortality); resistant to this
chemical at 1/3 sites (87% mortality); sensitive to deltamethrin
2/3 sites (mortality from 99% - 100%); resistant to this
chemical in Thach Dong Commune, Ha Tinh City (mortality
83%); sensitive to lambdacyhalothrin at 1/3 sites (100%
mortality); possibly resistant to this chemical at 1/3 sites (90%
mortality); resistant to this chemical at 1/3 sites (mortality


19
82%); possibly resistant to permethrin at 1/3 sites (mortality
95%); resistant to this chemical at 2/3 sites (mortality rate from

78% to 84%); sensitive to malathion at 2/3 sites (100%
mortality); resistant to this chemical in Thach Trung commune,
Ha Tinh city (mortality rate of 93%).
Susceptibility testing of Ae.albopictus at research sites in
Hanoi, Hai Phong, Thanh Hoa and Ha Tinh found that
Ae.albopictus was sensitive to alphacyhalothrin at 12/23
research sites; resistant to this chemical at 5/23 research sites;
and resistant to this chemical at 6/23 research sites; sensitive to
deltamethrin at 17/23 research sites; resistant to this chemical
at 1/23 research sites; resistant to this chemical at 5/23 research
sites; sensitive to lambdacyhalothrin at 10/23 research sites,
resistant to this chemical at 8/23 research sites, and resistant to
this chemical at 5/23 research sites; sensitive to permethrin at
16/23 research sites, resistant to permethrin at 4/23 research
sites and resistant to this chemical at 3/23 research sites;
Susceptible to malathion at 21/23 research sites, resistant to
this chemical at 1/23 research sites, resistant to malathion at
1/23 research sites. Different from Nguyen Van Dung (2011),
the study on mosquito, larva indices and insecticide
susceptibility of dengue vectors in some northern provinces of
Vietnam such as Thai Binh, Nam Dinh, Nghe An and Ha Tinh
showed that Ae.albopictus at most study sites was also
sensitive to deltamethrin, permethrin, malathion and resistant
to DDT. It was possibly resistant to alphacypermethrin,
lambdacyhalothrin. Thus, our results showed that
Ae.albopictus was still sensitive to most of the tested
insecticides:
Alphacypermethrin,
deltamethrin,
lambdacyhalothrin, permethrin, and malathion.

4.2. Correlation between climatic factors and dengue
situation in Hanoi
When analyzing the correlation between climatic factors
and Ae. aegypti index in Hanoi (2016 – 2017) we found that only
average monthly temperature was positively correlated with BI,
with r = 0.55 and statistically significant (p = 0.006). Rainfall,
average monthly humidity were correlated with BI but not
statistically significant. Meanwhile, average monthly temperature,
average monthly humidity and average monthly rainfall were
correlated with surface temperature but not statistically


20
significant. Our results showed a difference with the research of
Amarakoon (2007); Glenn L. Sia Su (2009); Hay (2002).
In addition to assessing the correlation between climatic
factors and vector indices, we also analyzed the correlation
between climate factors and vector indices and dengue cases in
Hanoi. Ambient temperature, average monthly rainfall and
mosquito density index with dengue cases in Hanoi had a
positive correlation r = 0.24, r = 0.19 and r = 0 , 39, but not
statistically significant with p = 0.27, p = 0.38 and p = 0.06;
The average monthly humidity and dengue cases were
inversely correlated with r = - 0.03, not statistically significant
with p = 0.89; Only BI and dengue cases had a positively,
closely correlated r = 0.66, which is statistically significant
with p = 0.00. Thus, only the BI index was correlated with the
number of dengue cases by months in Hanoi, somewhat similar
to that of Pham Van Hau (2011), with the study on ecological
factors. In the Central Highlands, Vietnam we found that after

seasonal adjustments, the risk of dengue fever was significantly
associated with the house index and BI.
When studying the correlation between climate factors,
vector 01 month ago and the dengue case of next month in
Hanoi, 2016 - 2017 found that: Temperature, average rainfall,
DM, BI, Case The disease of the previous month is correlated
with the case of dengue fever in the following month (p <0.05).
The results of the study coincide with a number of studies in
Vietnam and around the world such as Minh An (2014), the
study of epidemiological epidemiology in Hanoi and the
correlation with climate factors; Le Thi Thanh Xuan (2014), an
ecological research on climate change related to dengue cases
in Hai Phong; Phan Phuong Thao (2018) studies the threshold
of BI in predicting an outbreak of DF.

CONCLUSIONS
1. Distribution, resting habits, dengue transmission role
and insecticide susceptibility of Ae.aegypti and Ae.albopictus
in Hanoi, Hai Phong, Thanh Hoa and Ha Tinh, 2016 - 2017
- Distribution: Ae. aegypti và Ae.albopictus were found
in all studied provinces/cities, in which at most of the study
sites there was the presence of both the species, except for Hai
Phong and Thanh Hoa (only Ae. Aegypti found).


21
- Resting habits:
+ Ae. aegypti rested mainly indoors (86.5% to 96.2%),on
clothes (73.9% to 80.3%), in bedrooms (74.8% to 79.6%), at
the height of 1 - 2 meters (53.8% to 75.7%).

+ Ae.albopictus rested mainly outdoors (94.1% to
96.9%),on objects around larva sources (93.53% to 96.49%),at
the height of 1 - 2 meters (49.5% to 59.5%).
- Transmission role:
+ In active dengue outbreaks: there was the presence of
91.5% Ae. Aegypti, 60.7% Ae. Albopictus, and 52.1% both.
+ 9.35 % of the outbreaks had Ae. aegypti infected with
dengue virus; only 2.82% of Ae.albopictus were infected with
dengue virus.
+ Four dengue virus types were found including D1, D2, D3
and D4 infecting Ae. aegypti in Hanoi, D1 infecting Ae.
albopictus in Hai Phong.
- Insecticide susceptibility:
+ Ae.aegypti was resistant or possibly resistant to
pyrethroid chemicals: Alphacypermethrin, deltamethrin,
lambdacyhalothrin (resistant to alphacypermethrin and
deltamethrin at 9/13 research points, to Lambdacyhalothrin at
8/13 research points).
+ Ae.albopictus was mostly still sensitive to pyrethroid
chemicals (sensitive 12/23 - 16/23 research points) and
malathion (21/23 research points), except for populations in
Tan Trieu and Thanh Tri , Hanoi which were resistant to both
pyrethroid and malathion (with mortality from 3% - 73% in the
experiment).
2. Correlation between climatic factors, vector index and
the number of dengue cases in Hanoi, 2016 - 2017
- The average monthly ambient temperature and BI index
were proportionally correlated, r = 0.55, p = 0.006.
- BI and the number of dengue cases were proportionally
correlated, r = 0.66, p = 0.00

- Average temperature, mosquito density index of the
previous month and the number of dengue cases in the
following month in Hanoi were proportionally correlated, r =
0.48 and r = 0.49, with p = 0.02.


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- BI, the number of dengue cases of the previous month
and dengue cases of the following month were proportionally
correlated, r = 0.74 and r = 0.83, with p = 0.00.
- Humidity was not correlated with mosquito density and
BI, r = - 0.31 and r = - 0.22, p > 0.05.

RECOMMENDATIONS
1. The study on biological and ecological characteristics of
Ae. aegypti and Ae. albopictus should be continued to
determine any change in the future.
2. The efficacy of insecticides against Ae.aegypti and
Ae.albopictus should be continuously studied by biological
methods to select the appropriate chemicals for dengue
prevention and control.
3. To prevent and control dengue fever, attention should be
paid to the following factors: Average temperature, BI and
Ae.aegypti density and the number of cases to predict the
direction of dengue fever for the following month.